Conventionally, sheet processing apparatuses that perform various types of inspections of sheets have been put to practical use. A sheet processing apparatus has an image reading apparatus that reads an image of a sheet. The sheet processing apparatus takes in sheets that are placed in a feed unit in such a manner that one sheet is taken in at a time, and conveys the sheets to the image reading apparatus (light detecting apparatus). The sheets that are processed by the sheet processing apparatus may be banknotes, bills, or securities etc.
The image reading apparatus has an illumination unit (illuminating apparatus) and a line image sensor. The image reading apparatus irradiates a sheet with light from the illumination unit. The line image sensor of the image reading apparatus reads reflected light from the conveyed sheet and detects optical features (characteristics) of that sheet. The sheet processing apparatus identifies the sheet by comparing various preset parameters with the detected characteristics.
In cases where a sheet is conveyed at a high speed, there is a possibility that flapping (a variation in the position of the sheet in a direction perpendicular to a conveyance surface) may occur within a reading range of the image reading apparatus. Thus, there has been proposed an image reading apparatus having an illuminating apparatus that achieves increased uniformity of intensity of light in the direction perpendicular to the conveyance surface of a sheet by illuminating a reading target from both sides of a reading optical axis.
It is assumed that such sheet processing apparatuses detect creases, folds, and the like in a sheet. However, in cases where a sheet, which is the reading target, is illuminated from both sides of the reading optical axis by an image reading apparatus such as that described above, the entire surface of the sheet will be uniformly illuminated. Thus, creases and folds cannot cast shadows, and so characteristic images of the creases and the folds cannot be acquired. Accordingly, there is a problem in that the creases and the folds cannot be identified.